workloadattestor systemd: dbus use of closed network connection

Fixes spiffe#4315

The connection to system bus is shared and should not be closed after
use in order to avoid errors on concurrent usage.

It is typical to share the connection in the same process as per this
explanation:
godbus/dbus#179 (comment)

In case of errors, this shared connection will detect that and attempt
to reconnect: https://github.com/godbus/dbus/blob/v5.1.0/conn.go#L124.

pkg/agent/plugin/workloadattestor/systemd/systemd_posix.go

@@ -68,11 +68,11 @@ func (p *Plugin) Attest(ctx context.Context, req *workloadattestorv1.AttestReque
 }
 
 func getSystemdUnitInfo(ctx context.Context, pid uint) (*DBusUnitInfo, error) {
+	// Do not close this connection because it is shared and will autoclose on errors
 	conn, err := dbus.SystemBus()
 	if err != nil {
 		return nil, status.Errorf(codes.Internal, "failed to open dbus connection: %v", err)
 	}
-	defer conn.Close()
 
 	// Get the unit for the given PID from the systemd service.
 	call := conn.Object(systemdDBusInterface, systemdDBusPath).CallWithContext(ctx, systemdGetUnitByPIDMethod, 0, pid)